The next generation mobile wireless communication system (Fifth Generation (5G)), or New Radio (NR), will support a diverse set of use cases and a diverse set of deployment scenarios. The latter includes deployment at both low frequencies, i.e., 100s of Megahertz (MHz), similar to Long Term Evolution (LTE) today, and very high frequencies, i.e., millimeter (mm) waves in the tens of Gigahertz (GHz).
Codebook-Based Precoding
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a Multiple-Input Multiple-Output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The NR standard is currently being specified. A core component in NR is the support of MIMO antenna deployments and MIMO related techniques. NR will support uplink MIMO with at least 4 layer spatial multiplexing using at least 4 antenna ports with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation is provided in FIG. 1 for where Cyclic Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM) is used on the uplink (UL).
As seen, the information carrying symbol vector s is multiplied by an NT×r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and is typically indicated by means of a Transmit Precoder Matrix Indicator (TPMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same Time/Frequency Resource Element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
The received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modeled byyn=HnWsn+en where en is a noise/interference vector obtained as realizations of a random process. The precoder W can be a wideband precoder, which is constant over frequency, or frequency selective.
The precoder matrix W is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the User Equipment device (UE). In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
One example method for a UE to select a precoder matrix W can be to select the Wk that maximizes the Frobenius norm of the hypothesized equivalent channel:
      max    k    ⁢                                                H            ^                    n                ⁢                  W          k                            F    2  where                Ĥn is a channel estimate, possibly derived from SRS.        Wk is a hypothesized precoder matrix with index k.        ĤnWk is the hypothesized equivalent channel.        
In closed-loop precoding for the NR uplink, the Transmission Reception Point (TRP) transmits, based on channel measurements in the reverse link (UL), TPMI to the UE that the UE should use on its UL antennas. The NR base station (gNB) configures the UE to transmit Sounding Reference Signal (SRS) according to the number of UE antennas it would like the UE to use for UL transmission to enable the channel measurements. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be signaled.
Other information than TPMI is generally used to determine the UL MIMO transmission state, such as SRS Resource Indicators (SRIs) as well as Transmission Rank Indicators (TRIs). These parameters, as well as the Modulation and Coding State (MCS), and the UL resources where Physical Uplink Shared Channel (PUSCH) is to be transmitted, are also determined by channel measurements derived from SRS transmissions from the UE. The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder W. For efficient performance, it is important that a transmission rank that matches the channel properties is selected.
SRS Transmission in NR
SRSs are used for a variety of purposes in LTE, and are expected to serve similar purposes in NR. One primary use for SRS is for UL channel state estimation, allowing channel quality estimation to enable UL link adaptation (including determination of which MCS state the UE should transmit with) and/or frequency-selective scheduling. In the context of UL MIMO, they can also be used to determine precoders and a number of layers that will provide good UL throughput and/or Signal to Interference plus Noise Ratio (SINR) when the UE uses them for transmission on its UL antenna array. Additional uses include power control and UL timing advance adjustment.
Unlike LTE Release 14, at least some NR UEs may be capable of transmitting multiple SRS resources. This is similar conceptually to multiple Channel State Information Reference Signal (CSI-RS) resources on the downlink (DL): an SRS resource comprises one or more SRS ports, and the UE may apply a beamformer and/or a precoder to the SRS ports within the SRS resource such that they are transmitted with the same effective antenna pattern. A primary motivation for defining multiple SRS resources in the UE is to support analog beamforming in the UE where a UE can transmit with a variety of beam patterns, but only one at a time. Such analog beamforming may have relatively high directivity, especially at the higher frequencies that can be supported by NR. Earlier LTE uplink MIMO and transmit diversity designs did not focus on cases where high directivity beamforming could be used on different SRS ports, and so a single SRS resource was sufficient. When an NR UE transmits on different beams, the power received by the TRP can be substantially different. One approach could be to have a single SRS resource, but to indicate to the UE which of its beams to use for transmission. However, since UE antenna designs vary widely among UEs and UE antenna patterns can be highly irregular, it is infeasible to have a predetermined set of UE antenna patterns with which the TRP could control UE UL precoding or beamforming. Therefore, an NR UE may transmit on multiple SRS resources using a distinct effective antenna pattern on each SRS resource, allowing the TRP to determine the composite channel characteristics and quality for the different effective antenna patterns used by the UE. Given this association of each effective antenna pattern with a corresponding SRS resource, the TRP can then indicate to the UE which of one or more effective antenna patterns should be used for transmission on PUSCH (or other physical channels or signals) through one or more SRS resource indicators, or ‘SRIs’.
Non-Codebook Based Precoding
NR also supports non-codebook based transmission/precoding for PUSCH in addition to codebook based precoding. For this scheme a set of SRS resources are transmitted where each SRS resource corresponds to one SRS port precoded by some precoder selected by the UE. The gNB can then measure the transmitted SRS resources and feedback to the UE one or multiple SRIs to instruct the UE to perform PUSCH transmission using the precoders corresponding to the referred SRS resources. The rank in this case will be determined from the number of SRIs fed back to the UE.
By configuring the UE with the higher layer parameter SRS-AssocCSIRS and with the higher layer parameter ulTxConfig set to ‘NonCodebook’, the UE may be configured with a Non-Zero Power (NZP) CSI-RS to utilize reciprocity to create the precoders used for SRS and PUSCH transmission. Hence by measuring on the specified CSI-RS the UE will be able to perform gNB transparent precoding based on reciprocity.
Another mode of operation is to instead let the UE choose the precoders such that each SRS resource corresponds to one UE antenna. Hence, in this case the SRS resource would be transmitted from one UE antenna at the time and the SRIs would hence correspond to different antennas. Thus, by choosing the UE precoders like this the gNB will be able to perform antenna selection at the UE by referring to the different SRIs which in turn will correspond to different antennas.
As indicated above, non-codebook based precoding includes both antenna selection and gNB transparent reciprocity based precoding.
UE Coherence Capability in NR
Depending on UE implementation, it may be possible to maintain the relative phase of the transmit chains. In this case, the UE can form an adaptive array by selecting a beam on each transmit chain, and by transmitting the same modulation symbol on the selected beams of both transmit chains using a different gain and/or phase between the transmit chains. This transmission of a common modulation symbol or signal on multiple antenna elements with controlled phase can be labeled ‘coherent’ transmission’. The support for coherent uplink MIMO transmission in LTE Release 10 is indicated via a feature group indication for relative transmit phase continuity for UL spatial multiplexing, wherein a UE indicates if it can adequately maintain the relative phase of transmit chains over time in order to support coherent transmission.
In other UE implementations, the relative phase of the transmit chains may not be well controlled, and coherent transmission may not be used. In such implementations, it may still be possible to transmit on one of the transmit chains at a time, or to transmit different modulation symbols on the transmit chains. In the latter case, the modulation symbols on each transmit chain may form a spatially multiplexed, or ‘MIMO’, layer. This class of transmission may be referred to as ‘non-coherent’ transmission. Such non-coherent transmission schemes may be used by LTE Release 10 UEs with multiple transmit chains, but that do not support relative transmit phase continuity.
In still other UE implementations, the relative phase of a subset of the transmit chains is well controlled, but not over all transmit chains. One possible such example would be multi-panel operation, where phase is well controlled among transmit chains within a panel, but phase between panels is not well controlled. This class of transmission may be referred to as ‘partially-coherent’.
All three of these variants of relative phase control have been agreed for support in NR, and so UE capabilities have been defined for full coherence, partial coherence, and non-coherent transmission. Full coherence, partial coherence, and non-coherent UE capabilities are identified according to the terminology of Third Generation Partnership Project (3GPP) Technical Specification (TS) 38.331 Version 15.0.1 as ‘fullAndPartialAndNonCoherent’, ‘partialCoherent’, and ‘nonCoherent’, respectively. This terminology is used because a UE supporting fully coherent transmission is also capable of supporting partial and non-coherent transmission and because a UE supporting partially coherent transmission is also capable of supporting and non-coherent transmission. A UE can then be configured to transmit using a subset of the UL MIMO codebook that can be supported with its coherence capability. In 38.214 section 6.1.1, the UE can be configured with higher layer parameter ULCodebookSubset, which can have values ‘fullAndPartialAndNonCoherent’, ‘partialAndNonCoherent’, and ‘nonCoherent’, indicating that the UE uses subsets of a codebook that can be supported by UEs with fully coherent, partially coherent, and non-coherent transmit chains.
Antenna Ports
In TS 38.211 V15.0.0 section 6.3.1.5, the vector z corresponding to the antenna ports is specified for codebook based and non-codebook based precoding as follows:                The block of vectors        
            [                                    y                          (              0              )                                ⁡                      (            i            )                          ⁢                                  ⁢        …        ⁢                                  ⁢                              y                          (                              υ                -                1                            )                                ⁡                      (            i            )                              ]        T    ,      i    =    0    ,  1  ,  …  ⁢          ,            M      symb      layer        -    1                   shall be precoded according to        
      [                                                      z                              (                                  p                  0                                )                                      ⁡                          (              i              )                                                            ⋮                                                                z                              (                                  p                                      ρ                    -                    1                                                  )                                      ⁡                          (              i              )                                            ]    =      W    ⁡          [                                                                  y                                  (                  0                  )                                            ⁡                              (                i                )                                                                          ⋮                                                                              y                                  (                                      υ                    -                    1                                    )                                            ⁡                              (                i                )                                                        ]                      where i=0, 1, . . . , Msymbap−1, Msymbap=Msymblayer. The set of antenna ports {p0, . . . , pρ-1} shall be determined according to the procedure in [6, TS 38.214].        For non-codebook-based transmission, the precoding matrix w equals the identity matrix.        For codebook-based transmission, the precoding matrix w is given by w=1 for single-layer transmission on a single antenna port, otherwise by Tables 6.3.1.5-1 to 6.3.1.5-7 with the TPMI index obtained from the DCI scheduling the uplink transmission.        
UL Power Control
Setting output power levels of transmitters, base stations in DL, and mobile stations in UL in mobile systems is commonly referred to as Power Control (PC). Objectives of PC include improved capacity, coverage, improved system robustness, and reduced power consumption.
In LTE, PC mechanisms can be categorized into the groups (i) open-loop, (ii) closed-loop, and (iii) combined open- and closed loop. These differ in what input is used to determine the transmit power. In the open-loop case, the transmitter measures some signal sent from the receiver, and sets its output power based on this. In the closed-loop case, the receiver measures the signal from the transmitter, and based on this sends a Transmit Power Control (TPC) command to the transmitter, which then sets its transmit power accordingly. In a combined open- and closed-loop scheme, both inputs are used to set the transmit power.
In systems with multiple channels between the terminals and the base stations, e.g. traffic and control channels, different power control principles may be applied to the different channels. Using different principles yields more freedom in adapting the power control principle to the needs of individual channels. The drawback is increased complexity of maintaining several principles.
Power Control in NR
In TS 38.213 (V15.0.1), the UL power control for NR is specified in section 7 and it is specified how to derive PPUSCH,f,cc (i, j, qd, l) which can be described as the “output” from the UL power control framework; this is the intended output power that should be used by the UE to conduct PUSCH transmission. When performing PUSCH transmission it is specified in TS 38.213 section 7.1 that:                “For PUSCH, a UE first scales a linear value {circumflex over (P)}PUSCH,f,cc (i, j, qd, l) of the transmit power PPUSCH,f,cc (i, j, qd, l) on UL BWP b, as described in Subclause 12, of carrier f of serving cell c, with parameters as defined in Subclause 7.1.1, by the ratio of the number of antenna ports with a non-zero PUSCH transmission to the number of configured antenna ports for the transmission scheme. The resulting scaled power is then split equally across the antenna ports on which the non-zero PUSCH is transmitted.”        
As described herein, the inventors have found that the current UL power control scheme for PUSCH specified for NR has several problems. Solutions for addressing these problems are disclosed herein.